With technological advancement and demand for mobile instruments, demand for secondary batteries as an energy source is rapidly increasing. Among such secondary batteries, a lithium secondary battery having high energy density and working potentials, a long life cycle, and reduced self-discharge is widely available in the related art.
In addition, as environmental problems are increasingly of concern, a great deal of studies and investigation into an electric car and/or a hybrid car capable of replacing typical vehicles using fossil fuels such as gasoline, diesel, etc. are currently conducted. Such an electric and/or hybrid vehicle mostly uses a nickel metal hydride based secondary battery as a power source. However, a lithium secondary battery having high energy density and discharge voltage has also been actively studied in recent years and has partially entered commercialization.
A major cathode active material for the lithium secondary battery comprises lithium-containing cobalt oxide LiCoO2. Other substances including, for example, lithium-containing manganese oxide such as LiMnO2 with a layered crystalline structure or LiMn2O4 with a spinel crystalline structure, lithium-containing nickel oxide LiNiO2, and the like may also be considered.
Especially, although LiCoO2 with excellent lifespan and charge-discharge efficiency is widely used, the above material has some disadvantages such as low structural stability, high price due to cobalt as a limited mineral resource and, in turn, restriction in price competitiveness.
Lithium-manganese oxides such as LiMnO2, LiMn2O4, etc. have excellent thermal stability and economic merits, however, they entail some problems such as low capacity, poor high temperature characteristics, and so forth.
In addition, LiNiO2 active material is relatively cheap and has favorable battery characteristics with high discharge capacity. However, the foregoing active material shows a rapid phase transformation in crystalline structure caused by variation in volume during charge-discharge cycle and encounters a problem of drastically reduced stability when exposed to air and moisture.
In order to solve the above problems, the present invention is directed to provision of an economical cathode active material with high performance, including a lithium transition metal composite oxide in which each of constitutional elements has desired composition and oxidation number, as described below.
In this regard, U.S. Pat. No. 6,964,828 discloses a lithium transition metal oxide represented by Li(M1(1-x)-Mnx)O2 wherein M1 is a metal other than Cr and, if M1 is Ni or Co, all Ni in the oxide has an oxidation number of +2, all Co in the oxide has an oxidation number +3, and all Mn in the oxide has an oxidation number of +4.
Korean Laid-Open Application No. 2005-047291 discloses a lithium transition metal oxide containing Ni and Mn in equal proportions, wherein the oxidation numbers of Ni and Mn are +2 and +4, respectively.
Korean Patent No. 0543720 proposes a lithium transition metal oxide containing Ni and Mn in equal proportions, wherein the oxidation numbers are defined by Ni=2.0 to 2.5 and Mn=3.5 to 4.0, according to a typical method of measuring oxidation number. This means that Ni and Mn substantially have the oxidation numbers of +2 and +4, respectively. According to examples and comparative examples of the foregoing patent, it was described that the performance of the oxide may be deteriorated if the oxidation numbers of Ni and Mn are not +2 and +4, respectively.
Japanese Laid-Open Application No. 2001-00083610 suggests a lithium transition oxide represented by Li((Li(Ni1/2Mn1/2)(1-x))O2 or Li((Lix(NiyMnyCop)(1-x))O2, which contains Ni and Mn in equal proportions. Here, when Ni and Mn are substantially present in equal proportions, the oxide may include Ni2+ and Mn4+ and, in turn, structural stability, thereby obtaining a layered structure.
According to the above listed technologies, an average transition metal oxidation number may be +3. As an alternative example, U.S. Pat. No. 7,314,682 claims a compound represented by Li(2+2x)/(2+x)M′2x(2+x)/(2+2x)M(2−2x)/(2+x)O2-δ wherein M′ is an element with an average oxidation number of +3 except for Li, while M is a transition metal element with an average oxidation number of +3.
As disclosed in the above documents, provided that (i) a stable laminate structure is obtained when a lithium transition oxide includes transition metals with an average oxidation number of +3, the lithium transition oxide may exhibit superior electrochemical characteristics only when Ni and Mn are present in equal proportions and Ni has the average oxidation number of +2 while Mn has the average oxidation number of at least +4.
However, the present inventors found that, although a lithium transition metal oxide contains Mn and Ni in Mn4+ and Ni2+ states, respectively, so as to reach the oxidation number of +3, deterioration in electrochemical performance of the oxide caused by reversible migration of Ni2+ to a Li site cannot be overcome.
In addition, U.S. Pat. No. 6,660,432 proposes that a Co content of more than 10% and, preferably, 16% relative to a total amount of transition metals enables production of a well-grown crystalline structure and contents of Ni and Mn are substantially equal. However, if the Co content is too high, production costs are increased and Co4+ contained in transition metals during a charge state may be considerably unstable, thus reducing stability of the oxide.
U.S. Pat. Nos. 7,078,128 and 7,135,252 disclose substances containing more Mn than Ni. However, the present inventors found from experimental results that the oxidation number of Mn is unable to be varied during Li charging if Mn content is high, thereby reducing the capacity of a lithium transition metal oxide.